Negative electron affinity based photocathodes, often composed of group III-V elements, are used in many applications. In technological constructs, they are frequently employed as sensitive generators of photoelectrons fed into a cascade chain for signal amplification in photomultiplier tubes. Specialized negative electron affinity photocathode based tubes are used for low level light amplification in night-vision goggles and sights. Scientific applications include use as sources of spin-polarized and ultra-cold electrons. In all cases, the photocathode is activated to lower its electron affinity, thereby enabling photoelectrons to be emitted via excitation by relatively low energy visible and near infrared photons.
Activated photocathodes are restricted to operation in the very best ultra-high vacuum environments so that they exhibit stable operation over long periods. In sealed tubes, exposure is limited to gas generated from internal components through electron bombardment and heating. When used as a bolt-on electron source, the gas load may be compounded by connection to vacuum systems with higher pressures than the source. A major problem in the preparation and use of these photocathodes is the relatively high chemical reactivity of both the clean and activated III-V photocathode surfaces. There has been relatively little successful work to date to enhance the chemical immunity of activated III-V photocathodes. Earlier efforts to work around reactive gas susceptibility have included encapsulating the photocathode and overcoating with antimony. Both methods result in, at best, some decreased yield, and most problematic, a complicated preparation apparatus not easily integratable into existing systems. Prior to this invention, no satisfactory method had been developed so that GaAs and other III-V based photoemitters could be activated in a similar fashion to that currently employed in common practice while resulting in a surface that is significantly more stable against reactive gas driven photoyield decay.
GaAs photoemitters are activated to the negative electron affinity state by first starting with an atomically clean surface. Such a surface is obtained by chemical treatment, frequently followed by heating once the photoemitter has been introduced into a vacuum environment. Activation consists of the deposition of a low work function metal, such as a group IA alkali, followed or interleaved with an oxidizing agent onto the clean surface. The lowest affinities are obtained using Cs as the alkali and either oxygen or fluorine as the oxidizer. Low coverage of Cs is commensurate with the underlying lattice on some surfaces, but at the coverage required for activation, the layer is amorphous, due in part to the large covalent radius of the Cs atoms. Studies of the deposition process have shown that the initial oxygen absorption sites are between the Cs atoms. Together these facts suggest that if access to the underlying oxidizer and photocathode surface could be blocked during the activation process, but after the Cs and oxidizer atoms are in place, then absorbed gas induced decay could be inhibited. The covalent radius of the group IA elements decreases moving up the column from Cs to Li. A second, smaller covalent radius alkali could be used as blocking atoms if those atoms adsorb in an advantageous location and do not have a deleterious effect on the quantum yield.
A method based on two alkali photocathode activation would have great utility in all applications of lowered and negative electron affinity based photocathodes. Decreased lifetime sensitivity to reactive gas would enhance the shelf- and operational-lifetimes of photomultiplier and night vision tubes. Decreased sensitivity to reactive gas would allow the use of bolt-on negative electron affinity photocathode sources on systems where they are currently unable to operate due to the high reaction rate with many background gasses. The use of an activation process parallel to the current one that employs Cs alone would make integration into existing systems straightforward and convenient.